Referring to FIG. 1, a conventional down-conversion mixer includes a first single-ended to differential converting circuit 51, a second single-ended to differential converting circuit 52, a transconductance circuit 53, a mixing circuit 54 and an output unit 55. The first single-ended to differential converting circuit 51 converts a single-ended oscillatory voltage signal into a differential oscillatory voltage signal pair. The second single-ended to differential converting circuit 52 converts a single-ended input voltage signal of radio frequency into a differential input voltage signal pair. The transconductance circuit 53 converts the differential input voltage signal pair into a differential input current signal pair. The mixing circuit 54 mixes the differential input current signal pair with the differential oscillatory voltage signal pair to generate a differential mixed voltage signal pair of intermediate frequency. The output unit 55 amplifies and buffers the differential mixed voltage signal pair to generate a differential output voltage signal pair.
A noise figure (NF) of the conventional down-conversion mixer can be expressed by the following equation:
                                                        NF              =                            ⁢                              1                +                                  (                                                            NF                      52                                        -                    1                                    )                                +                                                                            NF                      53                                        -                    1                                                        G                    52                                                  +                                                                            NF                      54                                        -                    1                                                                              G                      52                                        ·                                          G                      53                                                                      +                                                                            NF                      55                                        -                    1                                                                              G                      52                                        ·                                          G                      53                                        ·                                          G                      54                                                                                                                                                              =                                ⁢                                                      NF                    52                                    +                                                                                    NF                        53                                            -                      1                                                              G                      52                                                        +                                                                                    NF                        54                                            -                      1                                                                                      G                        52                                            ·                                              G                        53                                                                              +                                                                                    NF                        55                                            -                      1                                                                                      G                        52                                            ·                                              G                        53                                            ·                                              G                        54                                                                                                        ,                                                          Equation        ⁢                                  ⁢        1            where NF52, NF53, NF54 and NF55 respectively denote noise figures of the second single-ended to differential converting circuit 52, the transconductance circuit 53, the mixing circuit 54 and the output unit 55, G52, G53 and G54 respectively denote power gains of the second single-ended to differential converting circuit 52, the transconductance circuit 53 and the mixing circuit 54, and G32<1.
The conventional down-conversion mixer occupies a relatively large area. Moreover, FIGS. 2, 3 and 4 respectively illustrate a conversion gain, the noise figure and isolation between input terminals of the conventional down-conversion mixer at which the single-ended oscillatory voltage signal and the single-ended input voltage signal are received, under a circumstance where the single-ended input voltage signal has a frequency within a range of 70 GHz to 100 GHz or within a range of 80 GHz to 100 GHz, and where the single-ended oscillatory voltage signal has a frequency lower than that of the single-ended input voltage signal by 0.1 GHz. It is known from FIGS. 2, 3 and 4 that, for the conventional down-conversion mixer, the conversion gain is relatively low, the noise figure is relatively high, and the isolation is relatively low.